is_equal(other:likeCurrent):BOOLEAN-- Is set equal to other? -- Do not take cursor positions, and capacity into
-- account, but equality_tester should be the same.-- (From GENERAL.)require
other_not_void: other /=Voidensure
consistent:standard_is_equal(other)impliesResult
same_type:Resultimpliessame_type(other)
symmetric:Resultimplies other.is_equal(Current)
same_count:Resultimplies count = other.count

symdif(other:DS_SET[G]) -- Add items of other which are not included
-- in current set and remove those which are.
-- (Use equality_tester's comparison criterion
-- if not void, use `=' criterion otherwise.) -- (From DS_SET.)require
other_not_void: other /= Void
same_equality_tester:same_equality_tester(other)